Galen Marek died saving the Rebel Leaders in THE FORCE UNLEASHED. His clone was created to finish the job of killing the rebels in TFU2, but we never knew the end of his story now that Disney has thrown out the EU.

When was he a teacher at Luke's Praxeum? The original plotline in the EU didn't involve him at the Yavin IV Academy. Where is your source?

Maleck, not Marek. That's like saying Darth MALAK was a master at the Academy because he had the same name. Perhaps there is a correlation between the characters (e.g. Starkiller's clone's son) but unless somewhere in the players guide or Lucasarts suggested it outside that one throw-away line, I sincerely doubt it implies any connection to Galen Marek.